In the microelectronics industry, a die carrying an integrated circuit is commonly mounted on a package carrier, such as a substrate, a circuit board or a leadframe, that provides electrical connections from the die to the exterior of the package. In one such packaging arrangement called flip chip mounting, the die includes an area array of electrically-conductive contacts, known as bond pads, that are electrically connected to corresponding area array of electrically-conductive contacts on the package carrier, known as solder balls or bumps. Typically, the solder bumps are registered with the bond pads and a reflow process is applied to create electrical connections in the form of solder joints between the die and the package carrier. The process of flip chip mounting results in a space or gap between the die and the package carrier.
The die and the package carrier are usually formed of different materials having mismatched coefficients of coefficient of thermal expansion. As a result, the die and the package carrier experience significantly different dimension changes when heated that creates significant thermally-induced stresses in the electrical connections between the die and the package carrier. If uncompensated, the disparity in thermal expansion can result in degradation in the performance of the die, damage to the solder joints, or package failure. As the size of the die increases, the effect of a mismatch in the coefficient of thermal expansion between the die and the substrate becomes more pronounced. In stacked die packages, the mismatch in coefficient of thermal expansion between the die laminate and the package may be even greater than in single die packages. The failure mechanism in stacked die packages may shift from solder joint damage to die damage.
To improve the reliability of the electrical connections in flip chip package assemblies, it is common in the microelectronics industry to fill the gap between the die and the package carrier with an encapsulant material. Underfilling with encapsulant material increases the fatigue life of the package and improves the reliability of the electrical connections by reducing the stress experienced by the electrical connections during thermal cycling or when the die and the package carrier have a significant temperature differential. The encapsulant material also isolates the electrical connections from exposure to the ambient environment by hermetically sealing the gap and lends mechanical strength to the package assembly for resisting mechanical shock and bending. The encapsulant material further provides a conductive path that removes heat from the die and that operates to reduce any temperature differential between the die and substrate. As a result, underfilling with encapsulant material significantly increases the lifetime of the assembled package.
Various conventional underfilling methods are used to introduce the encapsulant material into the gap between the die and the substrate. One conventional method relies surface tension wetting or capillary action to induce movement of a low-viscosity encapsulant material with strong wetting characteristics from a side edge into the gap. According to this method, encapsulant material is dispensed as an elongated single line, L-shaped or U-shaped bead adjacent to one, two or three contiguous side edges of the die, respectively, and capillary forces operate to attract the encapsulant material into the gap. Typically, the viscosity of the encapsulant material is reduced and the flow rate increased by pre-heating the substrate in the vicinity of the die to a uniform, steady-state temperature between about 40° and about 90°, before the encapsulant material is dispensed onto the substrate. The underfill material is subsequently cured after the electrical connections have been fully encapsulated.
With reference to FIG. 1, a time sequence for a typical underfilling operation relying on capillary action is shown. Isochronal contour lines 11 represent the advance of the leading edge or wave front of the encapsulant material 10 moving into the gap separating a die 12 from a substrate 14. Initially, the encapsulant material 10 is dispensed as an L-shaped bead onto the substrate 14 adjacent to contiguous side edges of the die 12 and is attracted into the gap by capillary forces. As time progresses, the wave front of encapsulant material 10 advances substantially diagonally, as indicated by arrow 16, through the gap. Drag causes the flow rate to diminish with increasing time as indicated by the reduced separations between adjacent pairs of contour lines 11 and, as the underfilling operation nears completion, the advance rate of the wave front of encapsulant material slows dramatically.
For larger size dies and smaller gap dimensions, the time necessary to underfill using conventional capillary underfilling methods becomes longer because of the longer fluid path of the liquid encapsulant and shear rates. As a result, throughput diminishes and underfilling operations become less cost effective. One way of enhancing the velocity of the encapsulant material is to perform a forced underfill that relies upon, for example, vacuum assistance to enhance the fill rate and the quality of filling. Vacuum-assisted underfilling utilizes a pressure differential created across a bead of encapsulant material to draw the encapsulant material into the gap. Regardless of the underfilling method, it is important that voids are not formed in the encapsulant material. Voids may result in corrosion and undesirable thermal stresses that degrade performance or adversely effect the reliability of the package assembly.
It would therefore be desirable to provide a manner of underfilling the gap formed between a die and a package carrier that prevents the occurrence of voids between the die and the package carrier and that reduces the time required to perform an underfilling operation.